Be aware that many USB printer cables (which often cost as much as a real commercial gamepad/keyboard/mouse encoder) do the USB to parallel conversion inside the giant "centronics" connector that plugs into the printer - the cable is just USB. You simply cannot just hook a button or two up to USB and expect it to do anything useful. USB just doesn't work that way. You can use a parallel port that way, and you can even use an old school RS-232 serial port that way, but USB doesn't work that way.
By far the easiest thing to do here is to buy a real encoder that does what you want. There are several good product lines out there, including the PAC series from Ultimarc and the Wiz series from GGG. There's plenty of documentation available, and they're actually designed for this usage.